It is well known that adding glass fibres to a plastic makes the plastic much stronger and less easily deformed. This composite material can be used to replace metals, but it has two problems. One is that it is rather less stiff, and the other is that it is more brittle, which means that it cracks too easily when hit, and when cracked can be too easily broken. The stiffness (or Young's modulus) can be improved if the glass fibres are replaced by carbon or boron fibres. Unfortunately, these composites are much more brittle than glass fibre composites.
It is also known that composites can be made more tough (i.e. less brittle) by (1) using tough fibres, or (2) by making the plastic matrix tougher, or (3) by sticking the matrix to the fibres less well, or (4) by using very large diameter fibres. Unfortunately, (1) tough fibres such as Kevlar (T.M.) do not impart such a high Young's modulus to the composite as do carbon or boron fibres; (2) making the matrix tougher has only a small effect on composite toughness; (3) reducing the adhesion between matrix and fibres reduces some important strengths of the composite (e.g. the shear strength); and (4) making large diameter fibres which are strong is extremely difficult and expensive. Thus, known procedures cannot be used to make economic composites which combine great toughness, high strength, and great stiffness. This subject has recently been critically reviewed in Proc. ICF4 1 557 (1977), G. A. Cooper and M. R. Piggott.